Protest organizers said a tractor blockade on the highway linking Athens to Greece's second largest city, Thessaloniki, would be in effect indefinitely unless the government withdrew proposals for tax hikes and an overhaul of the pension system.

Blockades on secondary roads, used by drivers to bypass the highway, were also being stepped up.

"They are asking farmers to pay higher taxes even if they have no income ... It's impossible," said Yiannis Zacharias, a blockade organizer in northern Greece.

The new austerity measures have prompted broad protests against Prime Minister Alexis Tsipras' year-old leftwing government, with strikes from a range of professional groups, from lawyers to ferry workers.

Greece is expected to slip back into a mild recession in 2016, while the global sell-off in financial markets saw the value of shares on the Athens Stock Exchange sink Monday to their worst level since 1989. The exchange's main index was down a further 2.7 percent in early afternoon trading Tuesday.

On Friday, farmers from across Greece are planning to travel to Athens, with plans to stage a three-day protest outside parliament.

Government spokeswoman Olga Gerovasili said Tsipras was willing to meet farming associations but insisted the "the negotiations cannot start from zero."

Farmers launched the tractor blockades on Jan. 20, and have since stepped up their protests along the nation's highways and at border crossings, causing long delays for drivers and truckers.

Police said Tuesday Bulgarian truck drivers used their vehicles to smash a customs boom barrier at the Greek border with Bulgaria. Four trucks crossed into Bulgaria, while a fifth truck was stopped and the driver arrested on public disturbance charges.

The conservative main opposition party called on the government to withdraw the pension overhaul plan to avoid "social upheaval."